1. Field of the Invention
The invention relates to a photo element, and more particularly to a photo element applied in an input display.
2. Description of the Related Art
In input displays, such as input liquid crystal displays, pixels generally comprise photo elements to serve as readout pixels with readout function, so that liquid crystal display panels can provide input and readout functions. The panels are referred to as in-cell input panels. Generally, photo elements are divided into two types: charge-mode photo elements and current-mode photo elements. FIG. 1 shows a circuitry of a conventional charge-mode photo element 100. The photo element 100 comprises a readout line Readout, a switch line SW, a bias line 30, a switch thin film transistor (TFT) 10, a photo TFT 20, and a capacitor C1. The photo TFT 20 is used to receive environment light. When a scan signal on the switch line SW turns on the switch TFT 10, the readout line Readout transmits charges to charge the capacitor C1 through the switch TFT 10. The brightness of the received environment light determines the conductivity of the photo TFT 20. When the scan signal on the switch line SW turns off the switch TFT 10 and the brightness of the received environment light is higher, the photo TFT 20 discharges the capacitor C1 more quickly. When the scan signal on the switch line SW turns off the switch TFT 10 and the brightness of the received environment light is lower, the photo TFT 20 discharges the capacitor C1 more slowly. As described above, light sensitivity of the photo TFT 20 seriously affects performance of the photo element 100. If the photo element 100 has high light sensitivity, the photo element 100 can operate in any light-receiving condition. Thus, it is important to enhance the light sensitivity of the photo TFT 20.
FIG. 2 shows a circuitry of a conventional current-mode photo element 200. The photo element 200 comprises a switch TFT STFT2, a photo TFT PTFT2, a readout line Readout2 (a first line), a switch line SW2 (a second line), and a bias line Bias1 (a third line). A first electrode (for example, drain D) and a gate G of the photo TFT PTFT2 are electrically coupled to the bias line Bias1, and a second electrode (for example, source S) thereof is electrically coupled to the switch TFT STFT2. A first electrode (for example, drain D), a second electrode (for example, source S), and a gate G of the switch TFT STFT2 are electrically coupled to the source S of the photo TFT PTFT2, the readout line Readout2, and the switch line SW2, respectively. When the switch line SW2 is at a high potential, the switch TFT STFT2 is turned on, and the photo TFT PTFT2 is turned on according to brightness of the received environment light to generate a corresponding light current. The corresponding light current can be provided to a signal detector (not shown in FIG. 2) through the switch TFT STFT2 and the readout line Readout2 for detecting a degree of the received environment light.
Moreover, FIG. 3 shows a circuitry of an improved photo element 300 of U.S. patent application Ser. No. 11/611,320. The photo element 300 comprises a switch TFT STFT1, a photo TFT PTFT1, a readout line Readout1 (a first line), and a switch line SW1 (a second line). A first electrode (for example, drain D), a second electrode (for example, source S), and a gate G of the switch TFT STFT1 are electrically coupled to a second electrode (for example, source S) of the photo TFT PTFT1, the readout line Readout1, and the switch line SW1, respectively. A first electrode (for example, drain D) and a gate G of the photo TFT PTFT1 are both electrically coupled to the switch line SW1. When the switch line SW1 is at a high potential, the switch TFT STFT1 and the photo TFT PTFT1 are turned on, and the photo TFT PTFT1 is turned on according to brightness of the received environment light to generate a corresponding light current. The corresponding light current can be provided to a signal detector (not shown in FIG. 3) through the switch TFT STFT1 and the readout line Readout1 for detecting degree of the received environment light. As the photo element 100 of FIG. 1, the light currents of the photo elements 200 and 300 of FIGS. 2 and 3 are affected by the brightness of the environment light received by the surfaces of the photo TFTs PTFT1 and PTFT2. Generally, the light current is directly proportional to the degree of the environment light received by the photo TFT. Thus, enhancing light sensitivity of the photo TFTs PTFT1 and PTFT2 is very important.